Field of the Invention
The invention relates to a high temperature fuel cell and to a high temperature fuel cell stack.
In a fuel cell stack formed of high temperature fuel cells, a contact layer, an electrolyte/electrode element, a further interconnecting conducting plate, etc. are disposed in that order on one another and below an upper interconnecting conducting plate which covers the high temperature fuel cell stack. A fuel cell stack is also abbreviated as a "stack" in the specialist literature. The electrolyte/electrode element in that case includes two electrodes and an electrolyte which is disposed between the two electrodes. The interconnecting conducting plates within the high temperature fuel cell stack in that case are constructed as bipolar plates. In contrast to an interconnecting conducting plate disposed at an edge of the high temperature fuel cell stack, those plates are provided on both sides with channels for supplying the electrolyte/electrode elements with a respective working medium.
In that case, each electrolyte/electrode element lying between two neighboring interconnecting conducting plates, including the contact layer directly adjoining the electrolyte/electrode element on both sides, and those sides of each of the two interconnecting conducting plates which adjoin the contact layer, together form a high temperature fuel cell.
That and other types of fuel cells are, for example, disclosed in a book entitled "Fuel Cell Handbook" by A. J. Appelby and F. R. Foulkes, 1989, pages 442 to 454, or an article entitled "Brennstoffzellen als Energiewandler" [Fuel Cells as Energy Converters], in Energiewirtschaftliche Tagesfragen, June 1993, Vol. 6, pages 382 to 390. As a rule, a high temperature fuel cell stack is composed of a large number of high temperature fuel cells. Accordingly, a large number of electrolyte/electrode elements must be connected with interconnecting conducting plates in a gas-tight manner, that is to say, in other words, integrally. The interconnecting conducting plates within the high temperature fuel cell stack, that is to say the bipolar plates, thus serve the purpose of connecting the electrolyte/electrode elements electrically in series with one another, and of supplying the electrodes of the electrolyte/electrode elements with working media. The interconnecting conducting plates for closing off the high temperature fuel cell stack differ from the bipolar plates in two ways. Those interconnecting conducting plates are provided only on one side with channels for supplying the electrolyte/electrode elements with working media, and the current produced in the high temperature fuel cells is led off from the high temperature fuel cell stack through those interconnecting conducting plates, rather than being fed to a further high temperature fuel cell.
High temperature fuel cells with a similar structure are further disclosed by German Published, Non-Prosecuted Patent Applications DE 44 06 276 A1, DE 41 32 584 A1, DE 39 35 722 A1, and DE 39 22 673 A1, by German Patent DE 44 00 540 C, and by European Patent Application 05 78 855.
In order to avoid mechanical stresses in the high temperature fuel cell stack, in particular during thermal cycles which unavoidably occur, materials for the interconnecting conducting plates have been developed which have an average coefficient of thermal expansion that approximately coincides with that of the electrolyte sheets. Ceramic materials have been developed in the LaCrO.sub.3 system, as well as metallic materials based on Cr. Both materials are expensive to obtain. Due to the large proportion of volume occupied by the interconnecting conducting plates in a high temperature fuel cell stack, the cost of the interconnecting conducting plates is a dominant factor in the total cost of the high temperature fuel cell stack.